Refactor `CallInfo` amongst Cranelift's backends

[alexcrichton]

This commit is aimed at making it easier to extend CallInfo amongst the backends with more fields. Most backends already share almost all the same fields except for minor differences so this commit moves backends to all roughly the same track for handling this to enable having one definition of CallInfo that is shared everywhere.

[alexcrichton]

Ok, as predicted size assertions got the best of me. I've refactored the other direction for backends to include the name/destination in the CallInfo uniformly across all backends.

That was a large-ish change though so mind taking another look @fitzgen?

[fitzgen]

I don't have time today, and I'm going backpacking next week, so while I don't mind taking a look when I get back you might want to find another reviewer :-p

[alexcrichton]

@elliottt would you be ok reviewing this?